Calcitonin is secreted by thyroid parafollicular cells (C cells) of mammals or by post-branchial body of vertebrates such as fishes and birds. It is a type of single-strand polypeptide compound containing 32 amino acid residues, and is one of important hormones to maintain calcium-phosphorus metabolism in vivo. It mainly takes the action of inhibiting bone absorption, and reducing blood calcium level. Although natural calcitonins isolated from different genera are somewhat different in terms of components of amino acid residues, all of them include in their molecules the following structural features: a dithio ring structure formed by cysteines at 1,7-positions of N-terminal and a proline amide group at C-terminal. Natural calcitonin as secreted by vertebrates such as fishes exhibits the highest activity, while that as secreted by mammals exhibits a relatively low activity, for example, the activity of salmon calcitonin (sCT, which first order sequence is:
H-cyclo-(Cys1-Ser-Asn-Leu-Ser5-Thr-Cys7)-Val-Leu-Gly-Lys11-Leu-Ser-Gln-Glu-Leu16-His-Lys-Leu-Gln-Thr21-Tyr-Pro-Arg-Thr-Asn26-Thr-Gly-Ser-Gly30-Thr31-Pro-NH2) is 30 times greater than that of human calcitonin (hCT). Calcitonin can effectively prevent osteoporosis, and can simultaneously alleviate the symptoms such as bone pain and anergy of the patients suffering from osteoporosis. At present, the synthetic analogues of sCT, hCT and eel calcitonin (eCT) are mainly used in clinic for the treatment of senile osteoporosis, postmenopausal osteoporosis, Paget's disease, hypercalcemia and bone pain caused by osteoporosis or bone tumor.
sCT, as developed by Sandoz Co., was mainly used for the treatment of osteoporosis of postmenopausal women and etc., and was also suitable for patients with estrogen contra-indications and male patients suffering from osteoporosis, and its injection had already been marketed in USA in 1986, with trade name Miacalcic, and a conventional dose of 10-20 μg/day. sCT could activate adenylate cyclase (cAMP). It was demonstrated by study that cAMP, as one important second messenger in osteoclasts, participated in the inhibitory action on osteoclasts. sCT could also act on human osteoblasts, to simulate the proliferation and differentiation of osteoblasts. The action of sCT of reducing calcium and phosphorus levels in blood was mainly effectuated by inhibiting the transformation of bone calcium to blood calcium. At the same time, sCT could also promote the excretion of calcium and phosphorus in urine and bile, and inhibit the absorption of calcium and phosphorus ions in digestive tract. In addition, sCT could also specifically bind with calcitonin receptor in cerebrum and hypothalamus, and mediate central analgesic effect.
However, natural sCT can be easily inactivated by enzymolysis in vivo, and has a relatively short action time, which therefore shall be administered parenterally. Natural sCT also has a relatively poor stability in solution as well as with enzyme, and this, someone thinks, may be relevant to its dithio ring. Thus, in clinic, natural sCT shall be administered by injection frequently for a long period of time in order to achieve the treatment effectiveness, which thereby results in a poor compliance of the patients with medical treatment, and a reduction in treatment quality. Moreover, due to the presence of dithio ring, the synthesis of calcitonin is relatively difficult, and the cost thereof is increased, so that the drug obtained is too expensive to accept by the patients.
It was proved by experiment that dithio ring took different actions with respect to the activity of different calcitonins Dithio ring in sCT was not an essential group for its activity, and the linear analogue, i.e., sCT analogue free of 1,7-dithio ring, could still retain a preferable bioactivity. The synthesis of linear salmon calcitonin analogue (hereinafter referred to as sCT(L)) would become less difficult, and the cost thereof would be reduced. In addition, it was demonstrated by study that polypeptide drugs, after PEGylated, could still retain a good bioactivity, and could have a notably prolonged half life in organism. At present, the PEGylation of sCT has already been reported. It was discovered, by studying the metabolism in kidney homogenate, that the metabolic half life of the products PEGylated at three sites was far higher than that of sCT (4.8 min), i.e., the metabolic half life of the product PEGylated at N-terminal was 125.5 min, that of the product PEGylated at Lys11 was 157.3 min, and that of the product PEGylated at Lys18 was 281.5 min (K C Lee, et al, Pharm. Res., 1999, 16:813-818). In addition, the site-specific PEGylation of sCT at 8-amino of Lys18 was also reported (Y S Youn, et al, Pharm. Dev. Technol., 2005, 10(3): 389-396; J Controlled Release, 2006, 114(3): 334-342; 2007, 117(3): 371-379).
Then, by virtue of the characteristics of PEG capable of prolonging the action time, increasing the bioavailability and etc. of peptide drugs, it is possible to carry out an exact site-specific mono-PEGylation of linear sCT analogues using a reaction-specific chemical modification process. Therefore, linear sCT analogues shall include, or to which shall be re-introduced, an amino acid residue comprising a reaction-specific functional group, to increase the specificity of the reaction. The preparation of the linear sCT analogues may be implemented by using any conventional technique in the art, preferably a chemical synthetic process.
The object of the present invention is to provide a type of PEGylated sCT(L) analogues, which is in favor of developing long-acting medicaments and preparations for the treatment of bone diseases, e.g., osteoporosis.